Positive displacement pumps are conventionally employed as fuel pumps for aircraft turbine engines in order to obtain sufficient fuel pressure for the engine during low speed starting conditions. Recent requirements to improve pump reliability, package size and weight have increased the need to employ centrifugal-type pumps in applications of this type. However, centrifugal hydraulic pumps, which rely upon high-speed rotation to obtain high output pressure, do not provide sufficient fuel pressure at the ten percent to fifteen percent speed range to permit engine starting.
System designs specifications typically require fuel pumps to operate at a specified flow rate with a vapor/liquid inlet ratio of 0.45, and with a net positive suction pressure or NPSP, which is the pressure at the pump inlet above true vapor pressure of the fuel, of 5 psi. Newer system specifications, however, require the 0.45 vapor/liquid inlet ratio capability over a wider engine flow range, and may even require a 1.0 vapor/liquid ratio with intermittent all-liquid or all-vapor operation. Furthermore, the NPSP requirements have been increased to 5 psi over the entire engine flow range, and in some cases even 3 psi over the engine flow range.
It is therefore a general object of the present invention to provide a rotary hydraulic machine of the subject type that provides sufficient output pressure for use during low speed starting of aircraft turbine engines and other applications of similar type, while retaining desirable features of centrifugal pumps in terms of reliability, package size and weight.
It is another object of the present invention to provide a rotary hydraulic pump that is capable of satisfying flow requirements in aircraft turbine engine fuel delivery systems over an extended engine operating range. A further object of the present invention is to provide a fuel pump of the describe character that is economical and efficient in construction in terms of the stringent weight and volume requirements in aircraft applications, and that provides reliable service over an extended operating lifetime.